12 Volt side of lifeBattery UniversityThings noted: You got 6 days of use out of a single battery, that is about typical. You have an "idiot light" battery monitor, it is built for that type of user, don't rely on it. Charging from the umbilical connector when a battery is below 12V, is a losing proposition, it's better to use jumper cables directly from the truck to the trailer battery. Depleting a battery below 12V is reducing the capacity of the battery. Discharging the battery to 8V is damaging the battery, sometimes to the point of no recovery. Almost all batteries sold at retail with the label "Marine" are not a purpose built deep cycle battery. For RV use, Reserve Capacity is meaningless, we use the 20 hr. discharge rate which is measured in Amp Hours. 50% of amp hour rate is the golden threshold of usage, do not exceed it.
The number one thing you can do and is easy, is to replace the incandescent lights with LED bulbs. If you're going to be primarily boondocking, a battery monitor is a good thing to have. A hand held multimeter is a must-have and inexpensive.
Good luck with your 12V system.